1. Field of the Invention
The present invention relates generally to driver integrated circuits, and more specifically, to a circuit for driving relays, light-emitting diodes (LED), or other low-voltage devices efficiently from a wide supply voltage range.
2. Background of the Invention
Relay drivers, lamp drivers and LED driver integrated circuits (ICs) are in common use for supplying a drive current or voltage to the above devices connected externally to an IC driver. Typically the IC requires a supply voltage in accordance with the drive voltage requirements of the external device, or comprises an open-collector/open-drain driver that provides a return path for a power supply connection made directly to the external device.
If a power supply in accordance with the requirements of the external device are not readily available, a higher supply voltage can be reduced by resistive dissipation, producing a low-cost but inefficient solution, or a lower or higher supply voltage can be adapted via a dedicated power converter, providing a more efficient, but higher-cost solution. When only a few external devices such as relays, LEDs or lamps are driven by one or more driver ICs, a dedicated power converter is typically not cost-justified. Also, when the power supply input can vary widely or may be powered from differing sources such as batteries, line voltage or rectified AC (such as in automotive, telecommunications, industrial control or home appliance applications), the above-described solutions may not be practical.
Further, when driving relays, the current required to maintain a state of the relay (hold-in current) is much less than the current required to switch the relay (pull-in current). Since the supplied current is generally determined by the voltage applied across the relay coil terminals, the typical solutions described above are further inefficient in that the current supplied to the relay coil over long periods of hold-in time is much greater than the minimum current required by the relay.
Last, when driving relays from a switching power supply converter, frequencies higher than 20 Khz (the upper range of human hearing) are typically used to avoid audible electromechanical noise from emanating from the relays. However, multiple relay installations using a separate power supply circuit for each relay can produce beat frequencies through mechanical mounting structures coupling the relays (such as printed circuit boards or chassis) producing audible mechanical noise.
Therefore, it would be desirable to provide an apparatus and method for driving low-voltage devices such as relays, lamps or LEDs efficiently from a wide range of power supply input voltages. It would further be desirable to provide a relay driver circuit that reduces or eliminates beat frequency mechanical noise in a multiple relay circuit.
The above objectives are achieved in a circuit for driving a low-voltage device from a widely varying input supply. The driver circuit comprises an inductor (which may be a relay coil) connected in parallel with a diode (which may be one or more LEDs coupled in series) a switch coupled to a common connection of the diode and the inductor, and a control circuit coupled to the control input of the switch for intermittently applying a current to the inductor whereby an energy level of the inductor/diode circuit is maintained during an activation period. The circuit may comprise multiple driver circuits having a synchronization input coupled to a common connection whereby a switching action of the multiple driver circuits is synchronized to avoid beat frequency mechanical noise from being generated between the low-voltage devices.